In recent years, bipolar transistors having high speed performance have been used in various fields of application, such as communication devices and storage systems. A bipolar transistor capable of increasing operation speed is the bipolar transistor using a selective epitaxial technology disclosed, for example, in JP-A No. 7-147287. FIG. 2 illustrates a p-type base layer 109 and an n-type type emitter region 111 formed in a patterned structure, wherein the width of the base layer 109 is determined by the emitter area 111. Since there is no requirement for considering alignment margin in the lithography disclosed, the collector-base junction area can be decreased in FIG. 2. However, in the bipolar transistor of this type, a parasitic capacitance derived from regions other than the collector-base junction is present.
In FIG. 2, a distance d1 is necessary between the base region 109 and the low concentration collector region 103b to account for misalignment in the lithographic steps, and the etching scattering that occurs upon forming an opening. Accordingly, a parasitic capacitance C1 is present between the base electrode 106 and the low concentration collector layer 103b, due to insulating film 105. The thickness of the collector-base isolation insulating film 105 is substantially equal to the base layer 109, and the base layer is reduced as the operation speed of the transistor is increased, thereby increasing the parasitic capacitance C1 as operation speed increases.
It is also necessary to provide a sufficient junction between the base layer 109 and the base electrode 106 to prevent an increase in the base resistance, and there is nonetheless a lower limit for the parasitic capacitance that is dependent on the external base-collector junction area. The parasitic capacitance increases as the transistor size is reduced, and becomes as large as 50% for the collector capacitance.
Thus, to increase the operation speed of a bipolar transistor, high cut off frequency, low base resistance and low collector capacitance are necessary. However, in existent bipolar transistors, the parasitic capacitance between the base and the collector inhibits high speed operation of the transistor. Further, while the reduction of transistor size and the decrease of the collector capacitance are essential to lower the consumption power of an operating transistor, the effect of the parasitic capacitance from the periphery of the transistor increases as the transistor size becomes smaller, thereby inhibiting the decrease in the consumption power.
Therefore, the need exists for a bipolar junction transistor having low consumption power, high speed, and low parasitic capacitance.